Electroluminescent device

ABSTRACT

An electroluminescent device ( 1 ) comprises a supporting substrate ( 2 ); at least two electrodes ( 3 ) positioned on the substrate ( 2 ); at least a three-dimensional percolated layer ( 4 ), positioned on the substrate ( 2 ) between the two electrodes ( 3 ), having a metallic mesoporous structure defining a multitute of cavities of micrometric or nanometric dimensions. Present in the cavities of the three-dimensional percolated layer ( 4 ) are a multitude of luminescent inclusions ( 5 ), which operate to emit light when energized by electrons which, as a result of electron tunneling, effect pass through the three-dimensional percolated layer ( 4 ).

[0001] The present invention relates to an electroluminescent device.

[0002] More specifically, the present invention proposes the productionof an electroluminescent device of novel conception, which isparticularly susceptible to be applied to the field of photonics and ison a competitive level with traditional electroluminescent devices, suchas LED and O-LED, both in terms of costs and attainable performances.

[0003] The object is attained, according to the present invention, by anelectroluminescent device having the characteristics of the attachedclaims, which are an integral part of the present description.

[0004] Further objects, characteristics and advantages of the presentinvention shall become apparent from the description hereunder and theattached drawings, provided purely as a non-limitative example in which:

[0005]FIG. 1 is a graphic representation of the potential barrierbetween a generic metal and the vacuum, in different conditions;

[0006]FIG. 2 is a schematic representation of an electroluminescentdevice produced in accordance with the present invention;

[0007]FIG. 3 is a schematic representation of an electroluminescentdevice produced in accordance with a first possible variant of thepresent invention;

[0008]FIG. 4 is a schematic representation of an electroluminescentdevice produced in accordance with a second possible variant of thepresent invention.

[0009] The electroluminescent device according to the invention is basedon the tunneling effect in a three-dimensional percolated layer.

[0010] A three-dimensional percolated layer is a metallic mesoporousstructure, composed of metallic nanoparticles interconnected with oneanother or dielectric metallic interconnections connected in such a wayas to guarantee electrical conduction; the interconnection or connectionmay be produced by tunneling, as will be explained hereunder. Accordingto the invention, the cavities of micrometric or nanometric dimensionswhich are found in the mesoporous structure house luminescentnanoparticles or macromolecules; as will be seen, these emit light whenthey are energized by the electrons which, as a result of tunneling,pass through the percolated layer.

[0011] The commonly accepted definition for mesoporous materialscomprises inorganic materials with pores with dimensions below 50 nm.Porous materials with pores of nanometric dimensions are the mostdifficult to produce. In particular, for orderly mesoporous materials“supramolecular templating” techniques are generally utilized, which useasymmetrical organic molecules as templates, to be removed once thenanoporous structure has been established. Metallic mesoporous materialscan instead be grown using evaporation techniques, such as thermalevaporation or electron beam evaporation.

[0012] With regard to tunneling effect, it must here be considered thatthe metal-insulator interface is a typical situation inside a metallicsystem at percolation level, which occurs at each discontinuity of thesystem.

[0013] There are various electron transport mechanisms through themetal-insulator interface, such as ohmic conduction, ionic conduction,thermal emission and field effect emission. In a given material, each ofthe aforesaid mechanisms prevails in a certain temperature and voltagerange (electric field) and has a characteristic dependence on thecurrent, the voltage and the temperature. These different processes arenot necessarily independent from one another.

[0014] Field emission, also called Fowler-Nordheim electron tunneling,consists in transporting electrons through a metal-insulator interfacedue to the passage, by tunneling effect, of the electrons from the Fermilevel of the metal to the conduction band of the insulator means.

[0015] This tunnel effect occurs when there are strong electric fields(hence the term “emission for field effect”) which are able to bend theenergy bands of the insulator means to form a narrow triangularpotential barrier between the metal and the insulator.

[0016]FIG. 1 provides for this object a schematic representation of thepotential barrier between a generic metal and the vacuum in threedifferent possible situations.

[0017] Generally, it is assumed that the potential energy of an electronpasses from zero inside the metal to the value E_(F)+Φ immediatelyoutside the surface of the metal. In FIG. 1 this case is represented bythe curve (a).

[0018] The potential barrier which an electron moving away from themetal encounters has instead a more gradual trend, as it is reasonableto think that initially the potential increases linearly with thedistance from the surface of the metal; when an electron reaches thedistance of a few Å from this surface it should feel the effect of anattractive force equivalent to the force due to a charge −e, in thepresence of which the potential energy of the electron may berepresented with a function of the type:${V(x)} = {\left( {E_{F} + \Phi} \right) - \left( \frac{e^{2}}{16\pi \quad ɛ_{0}x} \right)}$

[0019] where x represents the distance of the electron from the surfaceof the metal. In FIG. 1 this case is represented by the curve (b).

[0020] Finally, if an electric field is applied in the direction X inthe vacuum region surrounding the heated metal, the potential energy ofthe electron becomes of the type:${V(x)} = {\left( {E_{F} + \Phi} \right) - \left( \frac{e^{2}}{16\pi \quad ɛ_{0}x} \right) - {exE}}$

[0021] where E represents the electric field applied. By performing thederivative of this expression the presence of a maximum of the potentialbarrier is found, represented in FIG. 1 by the curve (c), which is foundat: $\quad\left\{ \begin{matrix}{x_{\max} = \left( {{e/16}\pi \quad ɛ_{0}E} \right)^{1/2}} \\{V_{\max} = {{V(x)} = {\left( {E_{F} + \Phi} \right) - \left( {e^{3}{E/4}\pi \quad ɛ_{0}} \right)^{1/2}}}}\end{matrix} \right.$

[0022] As can be seen in FIG. 1, the presence of an external electricfield produces a slight decrease in the effective work function. Thedecrease in the value of the typical work function of the metal in thevacuum is small if the external electric field is not very intense (upto the value of a few thousands of volts/meter): in this case themaximum potential is found at many Å of distance from the externalsurface of the metal. Even a small decrease in the value of Φ makes thephenomenon of thermal emission possible for many electrons withoutsufficient energy to pass over the potential barrier in the absence ofthe external electric field.

[0023] When the electric field becomes very intense, around 10⁹volts/meter, in addition to the decrease in the typical work function ofthe metal, the phenomenon of field effect emission or electron tunnelingalso occurs.

[0024] The potential barrier that is created at the metal-insulatorsurface becomes so thin that the electrons of the metal can pass throughit by quantum tunneling. At a critical value of the electric field thepotential barrier becomes thin enough and the electrons that are on theFermi level of the metal acquire a finite probability of passing throughit. For higher values of the electric field, the even thinner thicknessof the potential barrier allows electrons with even lower energies topass through by tunnel effect.

[0025] The current density of emission for field effect is strictlydependent on the intensity of the electric field, while it issubstantially independent from the temperature:$j \propto {E^{2}{\exp \left( {- \frac{b\quad \Phi}{E}} \right)}}$

[0026] where E represents the intensity of the electric field, Φrepresents the height of the potential barrier and b is a constant ofproportionality.

[0027] It is important to observe that, in the case of emission throughelectron tunneling, the electrons do not require thermal energizing (andthis explains the fact that j does not depend on the temperature), butan intense electric field that reduces the thickness of the potentialbarrier bending the conduction and valence bands of the insulator means.This explains the strict dependence of j on the intensity of theelectric field: in fact, in this case, the electrons do not pass overthe potential barrier but tunnel through it.

[0028] There should only be a slight probability of tunneling for Fermilevel electrons unless the barrier is thinner than 10 Å. Therefore, itis reasonable to expect that the critical value of the electric fieldabove which the phenomenon of emission through field effect will occuris about 3·10⁹ volts/meter. However, this type of emission also occurswith macroscopic electric fields up to 30 times less intense. It isprobable that local roughness in the surface of the metal is the causeof the presence of extremely intense electric fields, although only on alocal scale, and that the majority of the emission by field effect comesfrom these zones.

[0029] Inside a percolated metallic system, and specifically at eachmetal-vacuum interface, there are local increases in the electric fieldthat make it possible to reach the values of intensity of the electricfield required for electron tunneling to take place. It is important tostress that the smaller the dimensions involved in the field emissionphenomenon are, the greater the local increase in the electric field is.At each discontinuity of the percolated metallic system, where there isa local increase in the electric field and electron emission takes placeby field effect, a local increase in the current density should occur.In fact, just as those deriving from thermal emission, the electronsemitted by field effect contribute to the total electric current. Due tothis, the percolated metallic system should have a voltage-currentcharacteristic with non-ohmic trend: the increase in the current withthe voltage applied, thanks to the contributions of thermal emission andfield effect emission, should be faster than it is in an ohmic conductorwith linear characteristics.

[0030] In FIG. 2, the numeral 1 indicates as a whole anelectroluminescent device produced according to the precepts of thepresent invention, the operation of which is based on the concepts setforth above.

[0031] The device 1 has a “Current In Plane” architecture and is formedof several parts, namely:

[0032] a substrate, indicated with 2;

[0033] two lateral electrodes, indicated with 3;

[0034] a layer of metallic mesoporous material at percolation level,indicated with 4;

[0035] luminescent nanometric inclusions 5 in the layer of percolatedmaterial 4;

[0036] a transparent protective layer, indicated with 6.

[0037] The substrate 2 may be transparent and produced in common glass,prepared for example with an ultrasound cleaning process, or may beopaque and produced in plastic material. According to the invention,transparent substrates covered with special costly coatings, such asglass covered with ITO, used in O-LED, P-LED and liquid crystal devicetechnology, are not in any case required.

[0038] The lateral electrodes 3 are positioned on the glass substrate 2at the same level and are composed of a continuous metallic layer,deposited by evaporation; the metallic material utilized for the purposemay be copper, silver, gold, aluminum or similar.

[0039] The electric contact between the power generator, indicatedschematically with “Low V_(DC)”, of the electroluminescent device 1 andthe active layer of the device, composed by the layer 4 of metallicmesoporous material at percolation level, is established through theelectrodes 3.

[0040] At the ends of the layer 4, the electrodes 3 generate adifference of potential that induces tunneling of electric chargethrough this layer. If the voltage applied is high enough to create veryintense local electric fields (E ≈10⁷ V/cm), electron conduction bytunneling as previously described occurs inside the metallic layer 4 atpercolation.

[0041] The percolation point of a discontinuous metallic system isdefined as the point in which the film changes from acting as aninsulator, typical of the situation in which the film has a great numberof discontinuities in relation to the metallic islands, to act as aconductor, typical of the situation in which as the metallic islands arepredominant over the discontinuities in the film, direct “links” betweenits two ends are formed, in which conduction of electric current cantake place.

[0042] In a discontinuous metallic film at percolation level there aredifferent electron transport mechanisms. As mentioned, in addition tonormal ohmic conduction of the current, other transport mechanisms occurwhich involve the interface zones between the metal and thediscontinuities, in particular thermal emission and electron tunneling.

[0043] Thermal emission only occurs in discontinuous films forsufficiently high temperature values, while electron tunneling occursprevalently in films characterized by a large number of discontinuitiesof extremely small size, where sufficiently intense local electricfields form.

[0044] Evidence of the electron tunneling phenomenon is given by thenon-linear trend of the voltage-current characteristic shown bypercolated metallic systems. These show a current discharge that occursat a critical value of the applied voltage. The current discharge provesthat the conductibility of the system increases suddenly at the criticalvoltage value: this means that by applying suitable voltage, at thediscontinuities where sufficiently intense electric fields have beencreated, electron tunneling effect is obtained. The electrons extractedby the metallic islands towards the discontinuity zones contribute tothe total current that passes through the system, thus becomingresponsible for the current discharge which can be observed atmacroscopic level.

[0045] It is this very phenomenon which makes the percolated metallicsystem very interesting for the applications in an electroluminescentdevice. In fact, electron emission by the metallic islands by electrontunneling effect is used to energize the luminescent particles 5, forinstance in the form of semiconductor nanocrystals, metallicnanoparticles or molecules with phosphorescent properties, included inthe cavities of the percolated metallic layer 4.

[0046] The electrons extracted by the metallic islands by electrontunneling have sufficient energy to energize luminescence in theluminescent nanoparticles enclosed in the matrix composed of thepercolated metallic structure. The centers of luminescence withnanometric dimensions may be of various types. In particular they may beproduced by:

[0047] organic phosphoruses, that is luminescent organic molecules,evaporated together with the metallic structure, among which: Coumarin7, Alumnium-8-hydroxyquinoline, Spiro compounds, electroluminescentpolymers;

[0048] inorganic semiconductors (Si, CdSe, CdTe, “core-shell” CdSe/ZnSand CdSe/CdS structures), prepared with self-assembly techniques (whichallow control over the diameter of the particles), electrochemicaldeposition, Langmuir-Blodgett techniques; nanostructures of this typemay, if energized by incident electrons with a certain amount of energy,emit photons in the visible field and the near-infrared;

[0049] metallic nanocrystals (Au, Ag, Co, Ni, Pt, . . . ), prepared forexample chemically by reduction of metallic ions in solution orphysically by evaporation of the metal at high temperature; on thenanometric scale, these metals behave similarly to a semiconductor andare capable of emitting, if energized, visible photons or in thenear-infrared;

[0050] luminescent rare earths, such as metalorganic compounds ofeuropium, terbium (emission in the visible), erbium, ytterbium (emissionin the infrared).

[0051] The transparent protective layer 6 of the device 1 according tothe invention may finally be composed of very thin transparent glass(about 0.5 mm), produced with sol-gel process and deposited on thepercolated metallic layer 4 by spin-coating, dip-coating, evaporation orsputtering, or may be produced with another transparent plasticdielectric.

[0052] This protective layer 6 does not require the introduction of apolarization film, as required in O-LED technology, for which it isessential to increase the contrast of the output light. The protectivelayer 6 of the device 1 according to the invention, in addition to beingeasy to prepare and deposit, thus reduces the total cost of theproduction process.

[0053] In the case shown in FIG. 2, the metallic mesoporous material 4at percolation level is in the form of a single layer. In accordancewith a possible variant, shown schematically in FIG. 3, the effect ofextracting the electrons by the metallic islands which constitute thepercolated layer may be increased by replacing the single layer 4 ofFIG. 2 with a multi-layer percolated system.

[0054] The different layers may made of different metals or alternatelymetal/dielectric. In the first case, as shown in FIG. 3, all the layersof the system, indicated with 4A, must be at percolation level, toguarantee the same performances of electron transport obtained in thesingle layer, and must be distributed so as to be in direct contact withmetals with different work functions (or extraction potentials). In thesecond case, as shown in FIG. 4, the various layers 4A of metal atpercolation level must be alternated with discontinuous layers ofdielectric material, one of which is indicated with 4B. Thediscontinuity of the dielectric layers 4B is essential to guaranteeelectric conduction throughout the multi-layer system (and not througheach single metallic layer).

[0055] It is known that phenomena of electron emission by a metal,either due to thermal emission or electron tunneling, increase inintensity when atoms of an element characterized by a low work functionare distributed on the surface of a metal characterized by a high workfunction value, and vice versa. The multi-layer solution ensures theelectroluminescent device has an extremely vast contact area, whichincreases the possibilities of contact between metallic islands ofdifferent elements and contributes towards increasing the number ofelectrons extracted by tunneling effect. Combinations of metals forwhich electron emission by tunneling effect is possible for a fewElectronVolts applied to continuous electrodes are: Ca-Al, Ca-Ag, Ca—Cu,Ca—Au, Al—Au, Ag—Au.

[0056] The characteristics of the invention are clear from thedescription given. As well as increased stability, the advantages thenew electroluminescent device draws from the characteristics of thepercolated metallic layer include:

[0057] the possibility of obtaining light emission in both directions,as a metallic system at percolation level is almost completelytransparent;

[0058] the use of solutions with multi-layer of different layers ofdiscontinuous films has the advantage of increasing the total volumefrom which light is emitted.

[0059] It is clear to those skilled in the art that there are numerouspossible variants to the electroluminescent device described as anexample, without however departing from the scopes of intrinsic noveltyof the invention.

1. An electroluminescent device (1) comprising: a glass or plasticsupporting substrate (2); at least two electrodes (3) positioned on saidsubstrate (2); at least a three-dimensional percolated layer (4;4A)positioned on said substrate (2) between said electrodes (3), saidthree-dimensional percolated layer (4;4A) having a metallic mesoporousstructure defining a multitude of cavities with micrometric ornanometric dimensions, said structure being in particular composed ofmetallic interconnections or metallic dielectrics interconnectionsconnected so as to guarantee electric conduction; a multitude ofluminescent inclusions (5), in particular in the form of nanoparticlesor macromolecules, housed in respective cavities of saidthree-dimensional percolated layer (4;4A), where said luminescentinclusions (5) are operative to emit light when energized by electronswhich, as a result of electron tunneling effect, pass through saidthree-dimensional percolated layer (4;4A).
 2. Device according to claim1, characterized in that said electrodes (3) are operative to establishthe electric contact between an external power generator (Low V_(DC))and said three-dimensional percolated layer (4;4A), in order to generateto the ends of the latter a potential difference which induces transportof electric charge through the layer.
 3. Device according to claim 1,characterized in that it is provided with a protective layer (6) of saidthree-dimensional percolated layer (4;4A).
 4. Device according to claim1, characterized in that said substrate (2) is produced in glass orplastic material.
 5. Device according to claim 1, characterized in thatsaid electrodes (3) are composed of a respective continuous metalliclayer.
 6. Device according to the preceding claim, characterized in thatsaid continuous metallic layer is deposited by evaporation on saidsubstrate (2).
 7. Device according to claim 5, characterized in thatsaid metallic layer is composed of a material selected in the groupcomprising copper, silver, gold, aluminum, platinum and nickel. 8.Device according to claim 1, characterized in that said luminescentinclusions (5) are in the form of semiconductor nanocrystals, metallicnanoparticles or molecules with phosphorescent properties.
 9. Deviceaccording to claim 1, characterized in that said luminescent inclusions(5) are in the form of organic phosphoruses, such as Coumarin 7,Alumnium-8-hydroxyquinoline, Spiro compounds, electroluminescentpolymers.
 10. Device according to claim 1, characterized in that saidluminescent inclusions (5) are in the form of inorganic semiconductors,such as Si, CdSe, CdTe, “core-shell” CdSe/ZnS and CdSe/CdS structures.11. Device according to claim 1, wherein said luminescent inclusions (5)are in the form of metallic nanocrystals.
 12. Device according to claim1, characterized in that said luminescent inclusions (5) are in the formof luminescent rare earths, such as metalorganic compounds of europium,terbium, erbium and ytterbium.
 13. Device according to claim 3,characterized in that said protective layer (6) is made of glass oranother transparent plastic dielectric.
 14. Device according to thepreceding claim, characterized in that said glass is produced withsolgel process and deposited on said percolated metallic layer (4;4A) byspin-coating, by dip-coating, by evaporation or by sputtering. 15.Device according to claim 1, characterized in that it is provided with aplurality of three-dimensional percolated layers (4A).
 16. Deviceaccording to the preceding claim, characterized in that said layers (4A)are made of metals differing from one another or according to a repeatedlayout of the type metal-dielectric-metal-dielectric.
 17. Deviceaccording to claim 15, characterized in that said layers (4A) are madeof one metal alternated with discontinuous layers of dielectric material(4B).